Method of making xerographic plate by vacuum evaporation of selenium alloy



Sept. 16, 1969 v. E. STRAUGHAN 3,467,548

METHOD OF MAKING XEROGRAPHIC PLATE BY VACUUM EVAPORATION OF SELENIUMALLOY Original Filed Dec. 27, 1965 2 Sheets-Sheet 1 Z V" W M W 7?? 3-SENSITIVITY f f %9 26% w w I- 4 f2? 4% 44 442 AA$ 5 7.5 I75 |7.5 I75 2525 I 95 95 a2 e 82.5 s25 7s ezs ppm]: 0 o zoolooozooo 0 I00 10002000 o200 ppm BR 4 500 Fla 3 INVENTOR.

VIRGIL E. STRAUGHAN P 6, 1969 v. E. STRAUGHAN METHOD OF MAKINGXEROGRAPHIC PLATE BY VACUUM EVAPORATION OF SELENIUM ALLOY Original FiledDec. 27, 1965 2 Sheet$Sheet 2 INVENTOR. V I RGIL E. STRAUGHAN UnitedStates Patent 3,467,548 METHOD OF MAKlN G XEROGRAPHIC PLATE BY VACUUMEVAPORATION OF SELENIUM ALLOY Virgil E. Straughan, Euclid, Ohio,assignor, by mesne assignments, to Xerox Corporation, Rochester, N.Y., acorporation of New York Application Dec. 27, 1965, Ser. No. 516,529, nowPatent No. 3,312,548, which is a continuation-in-part of applicationSer. No. 293,357, July 8, 1963. Divided and this application May 31,1966, Ser. No. 571,150

Int. Cl. H011) 1/02; B44d 1/18; C23c 13/02 U.S. Cl. 117-217 4 ClaimsABSTRACT OF THE DISCLOSURE A method of making a xerographic plate whichcomprises vacuum evaporating a mixture of selenium, arsenic, and halogenonto an electrically conductive support member to form a photoconductivealloy layer thereon. The mixture comprises arsenic in a range of about0.5 to 50 percent by weight, a halogen in range of about to 10,000 partper million, with a balance of the mixture comprising selenium.

This application is a divisional application of applicants parentapplication Ser. No. 516,529 filed on Dec. 27, 1965, and now U.S. PatentNo. 3,312,548, which in turn is a continuation-in-part of applicants c0-pending. application Ser. No. 293,357 filed July 8, 1963, and nowabandoned.

This invention relates to Xerography and more specifically to a systemutilizing an improved photosensitive plate.

In the art of xerography, it is usual to form an electrostatic latentimage on a member or plate which comprises a substantially electricallyconductive backing member such as for example, a paper or a metallicmember, having a photoconductive insulating surface thereon. It haspreviously been found that a suitable plate for this purpose is ametallic member having a layer of vitreous selenium. Such a plate ischaracterized by being capable of receiving a satisfactory electrostaticcharge and selectively dissipating such charge when exposed to a lightpattern and, in general, is largely sensitive to light in the blue andblue-green spectral range.

In the usual form of xerography, the electrostatic charge pattern formedby the selective dissipation of charge noted above is converted into avisible image through the selective attraction of marking particles byknown methods of image development. The xerographic plate is accordinglyof great importance in the xerographic process as it is the elementresponsible for the creation of the charge pattern. Other forms ofxerographic plates are known including, for example, sheets of papercoated with a photoconductive mixture of zinc oxide particles in aninsulating resin. However, the vitreous selenium xerographic plateremains the most widely used because it is capable of holding anelectrostatic charge for long periods of time when not exposed to light,because it is relatively sensitive to light compared with otherxerographic plates, and because it has suflicient strength and stabilityto be reused hundreds or even thousands of times. The selenium plate,however, is susceptible to deleterious crystal growth when the plate isheated during operation. This growth of crystals in the selenium layerdestroys the photoconductive insulating properties of the selenium, andplaces a limit upon the eflective life of the selenium plate.

At the same time, improvements in the light sensitivity and response tolonger wavelengths are much desired. A significant contribution was madeby O. S. Ullrich in the U.S. Patent 2,803,542, which disclosed that theaddition of arsenic to selenium causes a general increase in the lightsensitivity of the xerographic plate and also causes the plate to besensitive to longer wavelengths of light.

There is still, however, a continuing need for plates which requirestill shorter exposure times and yield a wider range of reproduciblecolors.

It is, therefore, an object of this invention to provide an improvedselenium-arsenic xerographic plate and an improved method for preparinga selenium-arsenic xerographic plate which overcomes the above noteddisadvantages.

It is another object of this invention to provide a selenium-arsenicxerographic plate having increased light sensitivity.

It is a further object of this invention to provide a selenium-arsenicxerographic plate having a broadened range of spectral response.

It is yet a further object of this invention to provide an improvedmethod of making selenium-arsenic xerographic plates having increasedlight sensitivity and a broadened range of spectral response.

It is another object of this invention to provide a selenium-arsenicxerographic plate having improved thermal stability.

The foregoing objects and others are accomplished in accordance withthis invention by preparing a xerographic plate containing selenium,arsenic, and up to 10,000 parts per million (ppm) of at least one memberof the halogen family. In the preparation of this plate suitablequantities of selenium, arsenic, and .a halo gen are sealed in acontainer and reacted at an elevated temperature to form homogeneousmixture of these elements. The alloy is then cooled and applied to asuitable conductive supporting base by vacuum evaporation. When theevaporation process is completed, a finished plate is removed from thevacuum chamber.

In general, the effective range of arsenic in the selenium layer isabout 0.5 to 50 percent by weight with the preferred range being about 1to 25 percent. The lower limit of about 1 percent is dictated by thefact that arsenic in amounts as low as 0.5 percent raises thecrystallization temperature and 1 percent practically eliminatescrystallization. The upper limit of 25 percent is chosen because thisamount of arsenic in combination with halogen doping will achieveessentially the same degree of light sensitivity and broadened spectralresponse as As Se or 38.5 percent arsenic, without introducing the highdark discharge property of As Se The upper end of the preferred range,from 15 to 20 percent arsenic, would be more desirable from thestandpoint of obtaining the optimum light sensitivity.

The effective range of the halogen addition is from about to 10,000parts per million with about 100 to 5,000 parts per million beingpreferred. The sensitivity for a given amount of arsenic increases to acertain degree with increased amounts of the halogen. Although amountsof 10 parts per million do exhibit an increased sensitivity a moredesirable sensitivity value can be obtained with greater amounts, suchas at least 100 parts of the halogen. Similarly amounts as high as10,000 parts per million (1 percent) are effective, but are unnecessaryin most cases, in that there is no significant change over the use of5000 parts per million.

The selenium-arsenic-halogen composition may comprise the entireinsulation layer or be present as a thin outer layer overlaying a baselayer of pure selenium.

The advantages of the improved xerographic plate and the method forproducing said plate will become more apparent upon consideration of thefollowing disclosure of the invention; especially when taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of one form of xerographic plateaccording to the invention;

FIG. 2 is a schematic sectional view of a second form of xerographicplate according to the invention;

FIG. 3 is a bar graph showing the relative sensitivity of variousxerographic plates;

FIG. 4 shows the relative sensitivity of a selenium plate withincreasing amounts of arsenic, with and without the addition of ahalogen.

The halogen is primarily illustrated by the use of iodine in FIGURES 1to 4.

FIGURE 1 shows a first form of improved xerographic plate according tothe invention. Reference character 10 designates an electricallyconductive mechanical support member. This is conventionally a metalplate such as brass, aluminum, gold, platinum, steel or the like. Thesupport member may be of any convenient thickness, rigid or flexible, inthe form of a sheet, a web, a cylinder, or the like, and may be coatedwith a thin layer of plastic. It may also comprise such other materialsas metallized paper, plastic sheets covered with a thin coating ofaluminum or copper iodide, or glass coated with a thin layer ofchrominurn or tin oxide. An important consideration is that the memberbe at least somewhat electrically conductive or have a somewhatconductive surface and that it be strong enough to permit a certainamount of handling. Member 11 may even be dispensed with entirely insome cases, Reference character 12 designates the photoconductiveinsulating layer which is coated on member 11. As shown in the figure,this layer is comprised of vitreous selenium together with lesseramounts of arsenic and a halogen consisting of iodine, bromine, chlorineor fluorine. As shown in the xerographic art, this layer may be as thinas about 1 micron or as thick as about 300 microns or more, but for mostcommercial applications the thickness will generally lie between about20 to 80 microns. This range of 20 to 80 is preferred in that thethicker layers (i.e. those approaching 300 microns) show some signs lessdesirable adherence to the support member than lesser thickness. Asuitable method of making the plate of FIGURE 1 is described below forillustrative purposes only.

Suitable quantities of selenium, arsenic, and iodine are sealed in areaction vessel and reacted at 525 C. for three or four hours in arocking furnace. After the selenium mixture is cooled and removed fromthe reaction vessel, it is applied to a suitable support member 10, suchas a sheet of polished brass, by a vacuum evaporation process. Themixture is placed in a crucible within a bell jar and the brass plate issupported about 12 inches above the crucible and maintained at atemperature of about 70 C. The bell jar is evacuated to a pressure ofabout 5 l0 torr and the crucible is heated to evaporate the seleniummixture onto the support member 10. The evaporation typically takes fromabout 20 to 45 minues. When the evaporation process is completed thecrucible is permitted to cool off and the finished plate is removed fromthe bell jar.

FIG. 3 is a bar graph showing the relative sensitivity of platescorresponding to FIGURE 1 and prepared with different proportions ofarsenic and iodine or bromine. The sensitivities of a 100 percentselenium plate and a selenium-arsenic plate prepared under similarconditions are also included for reference. Sensitivity was measured byelectrostatically charging the various xerographic plates beneath acorona discharge element and then exposing the plates to light for of asecond, measuring the relative dissipation of charge by means of anelectrometer, and comparing this relative discharge with a seleniumcontrol plate. It is apparent from FIGURE 3 that the light sensitivityof xerographic plates prepared with arsensic and iodine exceed those ofa control plate made with ordinary selenium. The sensitivity of platesmade under similar conditions with 5, 17.5 and 25 percent arsensic andno iodine are included for purposes of comparison. It can be seen thatthe plates containing iodine exhibits a very substantial increase insensitivity as compared with the plates lacking iodine. This increasesensitivity may not become apparent until several days after the plateis made. The plate containing the bromine addition shows a sensitivitycomparable to plates containing iodine.

As shown in FIGURE 2 in a further embodiment of the invention, it hasbeen found advantageous to deposit the selenium-arsenic-iodine mixturein a thin surface layer layer on the xerographic plate onto a layer ofsubstantially pure selenium. A xerographic plate can be constructed inaccordance with this embodiment by including a second evaporationsource, such as a molybdenum boat, in the vacuum bell jar. The principalevaporation source is loaded with pure selenium or selenium with ironpowder while the selenium-arsenic-iodine mixture is placed in themolybdenum boat which comprises a second evaporation source. Theselenium is first evaporated onto a suitable support member 11 exactlyas described previously. As soon as the selenium evaporation iscompleted and without breaking the vacuum in the bell jar, theselenium-arsenic-iodine mixture is evaporated onto the selenium. Byconfining the arsensic and iodine to a thin surface layer, smallerquantities of these materials can be employed. In addition, the coatingprocess is simplified because the alloy is evaporated in a very shorttime and there is less concern about non-uniform distribution of thearsenic and iodine in the plate.

Layer 14 may be of any convenient thickness. Layers between about 0.1and 0.5 micron have been found satisfactory. There is, no upper limit onthis thickness, since as layer '14 becomes very thick, the embodiment ofFIG- URE 2 simply becomes the embodiment of FIGURE 1.

It is apparent that the layered embodiment is capable of producingsubstantially the same increase in speed as the plate incorporatingarsenic and selenium throughout the bulk of the selenium. There is anadditional benefit in employing the embodiment of FIGURE 2. Allxerographic plates tend to exhibit an increased rate of chargedissipation in darkness after being exposed to bright light. High speedplates tend to exhibit this undesirable property to a greater degree. Ithas been found that layered plates corresponding to FIGURE 2 exhibitthis effect, known as fatigue, to a lesser degree than thosecorresponding to FIGURE 1. Thus, after exposure to bright light, aselenium plate containing 10 percent arsenic and 100 parts per millionof iodine throughout its bulk lost over percent of its charge in 30seconds. When the same arsenic-selenium-iodine mixture was confined to aone-half micron surface layer the plate was able to retain more thanhalf its charge after the previous exposure to bright light. It may benoted that these two plates have nearly the same sensitivity.

It has been discovered that the spectral response of xerographic platesin accordance with the present invention is broadened in proportion tothe amount of arsenic present, as taught in U.S. Patent 2,803,542referred to previously. The addition of a halogen on the other hand,increases sensitivity without affecting the spectral respouse.

The increase in sensitivity for a given selenium plate with increasingamounts of arsenic, with and without a halogen, is shown in FIGURE 4. InFIGURE 4, curve A represents the light sensitivity of a selenium platewith increasing amounts of arsenic, with no halogen additive. The lightsensitivity of 1 at percent arsenic would be the sensitivity of a 100percent selenium plate. As can be seen from curve A, the sensitivity ofa seleniumarsensic plate does not show improved sensitivity over a 100percent selenium plate until the arsensic is added in amounts upward ofabout 13 percent. On the other hand, curve B shows that the addition of1000 parts per million of iodine increases the light sensitivity of theseleniumarsensic plate at any percentage of arsenic, reaching a maximumof about 6 times the sensitivity of a 100 percent selenium plate atabout 18 percent arsenic.

Iodine is the preferred halogen additive, in that it can be convenientlyadded as a solid in weighed amounts to arsenic and selenium in a Pyrexvial just prior to evacuation and sealing. As previously described, thevial is then heated in a rocking furnace to insure proper mixing andhomogenization.

Inasmuch as the other members of the halogen family are either liquid orgaseous at room temperature, additional precautions should be taken toinsure that they are properly combined with the selenium and arsenic.

Bromine is added as liquid drops from a burette to the arsenic andselenium which is precooled in a glass tube by a Dry Ice-acetonemixture. This procedure is important in order to prevent a complete lossof the bromine during evacuation since the melting point of bromine is 7C.

Chlorine may be added by slightly different procedure. In this procedurethe chlorine gas is admitted to an evacuating tube containing gramquantities of arsenic and selenium. The remaining standard amounts ofarsenic and selenium are added to the tube and cooled in Dry Iceacetonemixtures prior to sealing under vacuum. Both the bromine and chlorineare now sufiiciently blended with the arsenic and selenium and themixing and homogenization process is then carried out as set forth inthe description using iodine as an additive. It has been found thatbromine gives affects similar to iodine when used in the plates of thisinvention.

The following examples further specifically define the present inventionwith respect to the method of making the halogen-doped selenium-arsenicplates. The parts and percentages in the disclosure, examples, andclaims are by weight unless otherwise indicated. The examples below areintended to illustrate various preferred embodiments of making ahalogen-doped selenium-arsenic plate.

EXAMPLE I A mixture of about 17.5 percent arsenic, about 82.5 percentselenium, plus about 1000 p.p.m. of iodine are sealed in a Pyrex vialand reacted atabout 525 C. for about three hours in a rocking furnace.The mixture is then cooled to about room temperature, removed from thePyrex vial, and placed in a quartz crucible within a bell jar. Analuminum plate is supported about 12 inches above the crucible andmaintained at a temperature of EXAMPLE II A mixture of about 17.5percent arsenic and about 82.5 percent selenium are placed in a Prexvial. Bromine is added to this mixture in a concentration of about 500p.p.m. as liquid drops from a burette to the arsenic and seleniummixture which is precooled in the glass vial by a Dry Ice-acetonemixture. The Pyrex vial containing the resulting mixture is thenevacuated and sealed. The sealed Pyrex vial is then treated in the samemanner as the iodine-doped mixture set forth in Example I.

EXAMPLE III A mixture of about 15 percent arsenic and about percentselenium is mixed with chlorine by first placing one gram each ofarsenic and selenium in an evacuated tube. Chlorine gas is then admittedto the evacuated tube to produce a concentration of chlorine of about2,000 p.p.m. The chlorine reacts with the arsenic and selenium asevidenced 'by the evolution of heat. The remaining amounts of arsenicand selenium are then added to the tube and cooled in a Dry Ice-acetonemixture prior to sealing in the Pyrex vial under vacuum. The sealedPyrex vial is then treated in the same manner as the iodine-dopedmixture set forth in Example I.

The plates made in accordance with the present invention are normallyused in a xerographic process including at least the three basic stepsof charging, exposing, and developing. A plate, which has preferablybeen stored in darkness, is given a surface electrostatic charge bybeing passed under a corona discharge device or the like. A positivepotential or charge on the order of several hundred volts is typical.The plate is then exposed to a pattern of light and shadow, as in acamera. This selectively dissipates the charge previously applied andthe remaining charge forms a charge pattern conforming to the light Apattern. By using the plates of the present invention, substantialshorter exposure times are possible and a Wider range of colors may bereproduced due to the increased sensitivity and broadened spectralresponse of the plates. Finally, the electrostatic pattern is made intoa visible reproduction of the light pattern through selectiveelectrostatically controlled deposition of marking material. Apparatusand materials for carrying out these basic xerographic steps arewell-known in the art and need not be further described here.

Although special components and proportions have been stated in theabove description of the preferred embodiments of the seleniumxerographic plate, other suitable materials, as listed above, may beused with similar results. In addition, other materials may be added tothe mixture to synergize, enhance, or otherwise modify its properties.For example, the halogen may be conveniently added as a compound orarsenic or selenium. For example, sodium hypochlorite could well be asource of chlorine. This allows all the halogens to be added to theselenium-arsenic mixture as solids, notwithstanding the fact that somehalogens occur in their elemental form as a gas or liquid at roomtemperature.

What is claimed is:

1. A method of making a xerographic plate which comprises vacuumevaporating a mixture of selenium, arsenic and a halogen onto anelectrically conductive support member to form a photoconductive alloylayer thereon, said mixture comprising arsenic in the range of about 0.5to 50 percent by weight, a halogen in a range of about 10 to 10,000parts per million, with the balance comprising selenium.

2. The method of claim 1 in which the halogen comprises iodine.

3. A method of making a xerographic plate which comprises vacuumevaporating selenium onto an electrically conductive support member toform a vitreous selenium coating thereon, and during the final stages ofsaid evaporation step, evaporating a photoconductive alloy layer ontosaid vitreous selenium layer, said alloy layer comprising a mixture ofselenium, arsenic and a halogen, with said mixture comprising arsenic inthe range of about 0.5 to about 50 percent by weight, a halogen in therange of about 10 to 10,000 parts per million, with the balancecomprising selenium.

4. The method of claim 3 in which the halogen comprises iodine.

References Cited UNITED STATES PATENTS Hart 96-6 'Bixby et a1. 1l7-106 XParis 961.5 Ullrich 96-1.5

Bardeen 96-1.5 Blakney et a1 96-1.5

RALPH s. KENDALL, Primary Examiner A. GOLIAN, Assistant Examiner us. c1.X.R. 15 117-34, 106, 107, 201, 227

